1. Field of the Invention
This invention relates to a process for producing polyolefin materials of improved tensile strength and tensile modulus.
2. Prior Art
Polyolefins having molecular weights much higher than general-purpose polyolefins are known as ultra-high-molecular-weight polyolefins. Such polyolefins enjoy high credit as an engineering plastics for their superior resistance to impact and to abrasion and also for their peculiar self lubrication. These ultra-high-molecular-weight polyolefins are expected to be able to furnish molded products having high strength and high tensile modulus provided that they can be highly oriented. Therefore, there have been investigated a variety of methods of stretching ultra-high-molecular-weight polyolefins.
Paul Smith and Piet J. Lemstra et al. (Japanese Patent Laid-Open Publication No. 56-15408) proposed a process for stretching a certain gel derived from a decaline solution (dope) of an ultra-high-molecular-weight polyolefin at a high draw ratio. However, this dope can only be effected with limited polymer concentrations, say 3% by weight of a polyethylene having a weight-average molecular-weight of 1.5.times.10.sup.6 and 1% by weight of a polyethylene having a molecular weight of 4.times.10.sup.6. Such prior stretching methods are undesirable as dope preparation requires large amount of solvent and even meticulous care in dissolving the polymer in the solvent, entailing economical infeasibility and inconvenient handling.
It has been proposed to subject a dilute solution of an ultra-high-molecular-weight polyolefin in a solvent such as xylene, decaline and kerosene to isothermal crystallization by cooling thereby obtaining a mat accumulated with a single crystal for extrusion in solid state and stretching as disclosed for example in Japanese Patent Laid-Open Publication No. 59-187614, No. 60-15120 and No. 60-97836 and Preprints of the Society of Polymer Science, Japan, vol. 34, p. 873 (1985). This process is still uneconomical because of the large amount of solvent necessary for formation of single crystal mat.
It has also been proposed to stretch ultra-high-molecular-weight polyolefins without the use of solvents disclosed for example in Japanese Patent Laid-Open Publication No. 63-41512 and No. 63-66207 wherein a particulate ultra-high-molecular-weight polyolefin is subjected to compression molding at a temperature below its melting point and then to rolling and stretching, or as disclosed in Japanese Patent Laid-Open Publication No. 2-258237 wherein compression molding is effected continuously by means of a double belt pressure arrangement. The polyolefin products obtained by such stretching processes may be satisfactory in terms of mechanical strength and modulus, but is not quite acceptable in respect to elongation at break in some usages.
Japanese Patent Laid-Open Publication No. 59-100710 proposes to thermally contract the material after being stretched so as to provide enhanced toughness, but this process would lead to reduced strength and modulus of the resulting material.
It has now been found that there can be obtained a ultra-high-molecular-weight polyolefin material having a balanced proportion of tensile strength, tensile modulus and elongation at break by contracting the stretched molded article of the ultra-high-molecular-weight polyolefin under selected conditions and thereafter re-stretching the contracted article.